1. Field of the Invention
The present invention generally relates to a double cladding fiber and an optical fiber amplifier, in which a leaked pumping light is suppressed in the double cladding fiber as an amplifying medium.
2. Description of the Related Art
In a conventional double cladding fiber and a conventional optical fiber amplifier, a signal is amplified by injecting both the signal light to be amplified and a pumping light providing amplifying action, into an optical fiber being an amplifying medium at the same time. Conventionally, there are three schemes for injecting the signal light and the pumping light. The first one is a forward pumping scheme in which the signal light and the pumping light are injected to the same transmitting direction, and the second one is a backward pumping scheme in which the signal light and the pumping light are injected to the facing direction, and the last one is a bidirectional pumping scheme in which the pumping light is injected from both directions for the signal light.
In the forward pumping scheme in which the pumping light is injected in the transmitting direction of the signal light, or in the bi-directional pumping scheme in which the pumping light is injected from both directions, the signal light and the pumping light are output at the same time to an output end. This pumping light power simply acts as a noise for the signal light power. As a result, a signal to noise ratio (SNR) at the output end is deteriorated.
In order to suppress this kind of deterioration, generally a long wavelength pass filter (LWPF) or a filter called a pumping light cut filter is installed at the output end.
The amplifying medium widely adopted currently is called an erbium doped fiber (EDF) and its structure is composed of a core in which the light is transmitted and a clad covering the core. Furthermore, at the EDF, a high power outputting optical fiber amplifier has been developed, which is considered to be difficult to develop, and for this high power outputting optical fiber, an optical fiber called a double cladding fiber is used as this amplifying medium.
A conventional application can be found in Japanese Patent Application Laid-Open No. HEI 10-51050 which discloses an optical fiber for amplification in which a double cladding fiber for amplification and an optical fiber used generally for transmission are fused together easily. A further conventional application can be found in Japanese Patent Application Laid-Open No. HEI 10-5106 disclosing an optical fiber for amplification in which the influence of a micro-bending loss is reduced and the connecting process of the end of the optical fiber is made to be easy.
However, in the conventional double cladding fibers mentioned above, a plurality of optical fibers are used as the amplifying medium. As a result, there is a problem that the structure becomes complex, in which a double cladding fiber is used with a pumping light cut filter having high efficiency.
It is therefore an object of the present invention to provide a double cladding fiber and an optical fiber amplifier in which an equivalent function to a pumping light cut filter having high efficiency is realized with a simple structure.
According to a first aspect of the present invention, for achieving the object mentioned above, there is provided a double cladding fiber, which includes a core, whose refractive index is n7a, transmits a signal light, a first clad, whose refractive index is n7b, covers the surrounding surface of the core, and a second clad, whose refractive index is n7c, covers the surrounding surface of the first clad. A designated length of the second clad is removed, and a designated substance, whose refractive index is xe2x80x9cnxe2x80x9d (xe2x80x9cnxe2x80x9d greater than n7b), is coated at the part [removed said where the second clad is removed on the first clad.
According to a second aspect of the present invention, in the first aspect, the refractive indices have a relation that n7a greater than n7b greater than n7c. 
According to a third aspect of the present invention, in the first aspect, a small amount of rare-earth element is doped in the core.
According to a fourth aspect of the present invention, there is provided an optical fiber amplifier, which includes a wavelength division multiplex (WDM) coupler, to which a signal light and a pumping light are inputted, an isolator from which an amplified signal light is outputted, and a double cladding fiber, which connects between the WDM coupler and the isolator, and consists of a core, whose refractive index is n7a, a first clad, whose refractive index is n7b, covering the surrounding surface of the core, and a second clad, whose refractive index is n7c, covering the surrounding surface of the first clad, and a designed substance, whose refractive index is xe2x80x9cnxe2x80x9d (xe2x80x9cnxe2x80x9d greater than n7b), is coated at the part removed from the second clad on the first clad.
According to a fifth aspect of the present invention, in the fourth aspect, said refractive indices have a relation that n7a greater than n7b greater than n7c. 
According to a sixth aspect of the present invention, in the fourth aspect, a small amount of rare-earth element is doped in the core.